BlockEntities

BlockEntities are like simplified Entities that are bound to a Block. They are used to store dynamic data, execute tick based tasks, and dynamic rendering. Some examples from vanilla Minecraft would be handling of inventories on chests, smelting logic on furnaces, or area effects on beacons. More advanced examples exist in mods, such as quarries, sorting machines, pipes, and displays.

Note

BlockEntities aren’t a solution for everything and they can cause lag when used wrongly. When possible, try to avoid them.

Creating a BlockEntity

In order to create a BlockEntity, you need to extend the BlockEntity class. To register it, listen for the appropriate registry event and create a BlockEntityType: 

@SubscribeEvent
public static void registerTE(RegistryEvent.Register<BlockEntityType<?>> evt) {
  BlockEntityType<?> type = BlockEntityType.Builder.of(supplier, validBlocks).build(null);
  type.setRegistryName("mymod", "mybe");
  evt.getRegistry().register(type);
}
In this example, supplier is a BlockEntityType$BlockEntitySupplier that creates a new instance of your BlockEntity. A method reference or a lambda is commonly used. The variable validBlocks is one or more blocks (BlockEntityType$Builder#of is varargs) that the block entity can exist for.

Attaching a BlockEntity to a Block

To attach your new BlockEntity to a Block, the EntityBlock interface must be implemented on your Block subclass. The method EntityBlock#newBlockEntity(BlockPos, BlockState) must be implemented and return a new instance of your BlockEntity.

Storing Data within your BlockEntity

In order to save data, override the following two methods: 

BlockEntity#save(CompoundTag tag)

BlockEntity#load(CompoundTag tag)
These methods are called whenever the LevelChunk containing the BlockEntity gets loaded from/saved to a tag. Use them to read and write to the fields in your block entity class.

Note

Whenever your data changes, you need to call BlockEntity#setChanged; otherwise, the LevelChunk containing your BlockEntity might be skipped while the level is saved.

Important

It is important that you call the super methods!

The tag names id, x, y, z, ForgeData and ForgeCaps are reserved by the super methods.

Ticking BlockEntities

If you need a ticking BlockEntity, for example to keep track of the progress during a smelting process, another method must be implemented and overridden within EntityBlock: EntityBlock#getTicker(Level, BlockState, BlockEntityType). This can implement different tickers depending on which logical side the user is on, or just implement one general ticker. In either case, a BlockEntityTicker must be returned. Since this is a functional interface, it can just take in a method representing the ticker instead:

// Inside some Block subclass
@Nullable
@Override
public <T extends BlockEntity> BlockEntityTicker<T> getTicker(Level level, BlockState state, BlockEntityType<T> type) {
  return type == MyBlockEntityTypes.MYBE.get() ? MyBlockEntity::tick : null;
}

// Inside MyBlockEntity
public static void tick(Level level, BlockPos pos, BlockState state, T blockEntity) {
  // Do stuff
}

Note

This method is called each tick; therefore, you should avoid having complicated calculations in here. If possible, you should make more complex calculations every X ticks. (The amount of ticks in a second may be lower then 20 (twenty) but won’t be higher)

Synchronizing the Data to the Client

There are three ways of syncing data to the client: synchronizing on chunk load, on block updates, and with a custom network message.

Synchronizing on LevelChunk Load

For this you need to override 

BlockEntity#getUpdateTag()

IForgeBlockEntity#handleUpdateTag(CompoundTag tag)
Again, this is pretty simple, the first method collects the data that should be sent to the client, while the second one processes that data. If your BlockEntity doesn’t contain much data, you might be able to use the methods out of the Storing Data within your BlockEntity section.

Important

Synchronizing excessive/useless data for block entities can lead to network congestion. You should optimize your network usage by sending only the information the client needs when the client needs it. For instance, it is more often than not unnecessary to send the inventory of a block entity in the update tag, as this can be synchronized via its AbstractContainerMenu.

Synchronizing on Block Update

This method is a bit more complicated, but again you just need to override two methods. Here is a tiny example implementation of it: 

@Override
public ClientboundBlockEntityDataPacket getUpdatePacket(){
    CompoundTag tag = new CompoundTag();
    //Write your data into the tag
    return new ClientboundBlockEntityDataPacket(getBlockPos(), -1, tag);
}

@Override
public void onDataPacket(Connection connection, ClientboundBlockEntityDataPacket pkt){
    CompoundTag tag = pkt.getTag();
    //Handle your Data
}
The Constructor of ClientboundBlockEntityDataPacket takes:

  • The position of your BlockEntity.
  • An id, though it isn’t really used besides by Vanilla; therefore, you can just put a -1 in there.
  • A CompoundTag which should contain your data.

Now, to send the packet, an update notification must be given on the server. 

Level#sendBlockUpdated(BlockPos pos, BlockState oldState, BlockState newState, int flags)
The pos should be your BlockEntity‘s position. For oldState and newState, you can pass the current BlockState at that position. flags is a bitmask that should contain 2, which will sync the changes to the client. See Constants$BlockFlags for more info as well as the rest of the flags. The flag 2 is equivalent to Constants$BlockFlags#BLOCK_UPDATE.

Synchronizing Using a Custom Network Message

This way of synchronizing is probably the most complicated but is usually the most optimized, as you can make sure that only the data you need to be synchronized is actually synchronized. You should first check out the Networking section and especially SimpleImpl before attempting this. Once you’ve created your custom network message, you can send it to all users that have the BlockEntity loaded with SimpleChannel#send(PacketDistributor$PacketTarget, MSG).

Warning

It is important that you do safety checks, the BlockEntity might already be destroyed/replaced when the message arrives at the player! You should also check if the chunk is loaded (Level#hasChunkAt(BlockPos)).